Is hydrodynamic escape from Titan possible?

When examining thermal atmospheric escape, usually either Jeans escape or hydrodynamic escape is considered. Jeans escape, where particles with velocities higher than the escape velocity can escape a planetary atmosphere, is usually considered, when particles in a collision-free region are examined. Hydrodynamic escape, on the other hand, presumes that the outflowing gas can be considered as a continuous, homogeneous medium where neither light, nor heavy particles can be discriminated from each other. Recently, Strobel (2009) applied a so-called ‘slow hydrodynamic escape model’, which describes cases intermediate between Jeans escape and hydrodynamic escape, for the nitrogen and methane molecules in Titanʹs upper atmosphere. This model requires an extended quasi-collisional region above the exobase where efficient energy transfer can presumably occur. In this study, we examine the collision probability of nitrogen and methane molecules with ambient atmospheric particles within Titanʹs exosphere using a modified Monte Carlo code introduced by Wurz and Lammer (2003), to analyze if the ‘slow hydrodynamic escape model’ is applicable to Titanʹs exosphere or not. Our results show that the collision probability of nitrogen and methane within Titanʹs exosphere decreases quickly with height above the exobase. Also, the probability of a nitrogen or a methane molecule to collide with another heavy molecule is far larger than the probability of a collision with a light particle, since in the region where nitrogen and methane are mainly present, the heavy molecules dominate the light molecules by a factor of 10–100. The results of our particle simulation do not confirm the existence of an extended quasi-collisional region above the exobase, where heavy, slow molecules can gain the escape velocity through collisions with light, fast particles.